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Distillation is a method of separation of substances based on differences in their volatilities. It is a chemical unit operation (mass transfer). Some may not notice, but distillation is actually a very important in our modern civilization. Distillation is used to separate crude oil into more fractions for specific uses such as transport, power generation, heating. Air is distilled to its components such as oxygen for medical applications and helium for party balloons. Known since antiquity, the concentration of alcohol by the application of heat to a fermented liquid solution is perhaps the oldest form of distillation, in the course of producing distilled beverages. An analogous method of purification using freezing instead of evaporation is called freeze distillation. It is not distillation, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively. Dry distillation, despite its name, is not distillation, but rather a chemical reaction known as pyrolysis in which solid substances are heated in a strongly reducing atmosphere and any volatile fractions collected. History Distillation was developed into its modern form with the invention of the alembic by Islamic alchemist Jabir ibn Hayyan c. 800; he is also credited with the invention of numerous other chemical apparatus and processes that are still in use today. The design of the alembic has served as inspiration for some modern micro-scale distillation apparatus such as the Hickman stillhead. As alchemy evolved into the science of chemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to condense the distillate and let it drip downward for collection. Applications of distillation The application of distillation can roughly be devided in three groups: laboratory scale, industrial distillation and food processing. The latter of the three is distincively different from the former two, in that in the processing of e.g. beverages the distillation is not used as a true purification method, but more to transfer all volatiles from the source materials to the distillate. The main difference between laboratory scale distillation and industrial distillation can be found in that laboratory scale distillations are often run batch-wise. In batch distillation the composition of the source material, the vapors of the distilling compounds and the distillate vary during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, then the mixture is separated into its component fractions which are collected sequentially from most volatile to less volatile, and the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated. In industrial applictations, distillation is often run in an equilibrium state (continuous distillation), where the system is kept at a constant composition by careful (balanced) replenishing of source material and removal of (enriched) fractions from both vapor and liquid phase in the system. This results in a better control of separation due to the constant composition of both gas and liquid phase. Theory It is a common misconception that in a solution, each component boils at its normal boiling point - the vapors of each component will collect separately and purely. This does not occur even in an idealized system. Idealized models of distillation are essentially governed by Raoult's law and Dalton's law. Raoult's law assumes that a component contributes to the total vapor pressure of the mixture in proportion to its fraction of the mixture and its vapor pressure when pure. For component A, PA = XAPA° where XA denotes the mole fraction of A and PA° denotes the vapor pressure of pure A. If a component changes another's vapor pressure, or the volatility of a component is dependent on its fraction, the law will fail. Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture. Ptotal = Σ Pi, for components i = A, B, C, ... Vapor pressures increase with heat. When a multi-component system is heated, the vapor pressure of each component will rise, causing the total vapor pressure to rise in turn. When the total vapor pressure reaches the ambient pressure, boiling occurs and liquid turns to gas throughout the bulk of the solution. Notice that a given mixture has one boiling point, when the components are mutually soluble. The idealized model is accurate in the case of chemically similar liquids, such as benzene and toluene. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate vapor-liquid equilibrium data is by measurement. By the nature of the process, it is theoretically impossible to completely purify the components by using distillation, as distillation only tends to approach purity, never reaching it. This is comparable to dilution, which never reaches purity. If ultra-pure products are the goal, then further chemical separation must be applied. The case of batch distillation Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (as for example in an apparatus depicted in the figure in the top, right) until boiling results in a gas-phase above the liquid which contains a mixture of A and B. The ratio between A and B in the gas phase will be a different ratio from the ratio in the liquid phase: the ratio in the liquid phase is determined by how the original mixture was prepared, the ratio in the gas phase follows from Raoults Law, vide supra, it will be enriched in the more volatile compound, A. Since we have not removed anything from this system (yet), the total ratio in the system is the same as our starting ratio. From that follows, that the ratio in the liquid has already changed from the starting ratio (i.e. a bit richer in B than the starting liquid). If we heat on, the temperature of the system will rise, resulting in the gas-phase rising in the setup, until it reaches the thermometer. At this point, the gas-phase will condense not only back into the flask, but also into the condenser. In other words, we are removing a bit of the mixture from our system. The destillate will be enriched in component A (it will have the same composition as the gas-phase, at ambient pressure and at the temperature indicated by the thermometer). The result is thus, that the ratio in the boiling gas/liquid mixture is changing, it is becoming richer in component B. Therefore the boiling point of the mixture is rising, which again results in a rise in the temperature in the gas phase, which results in a changing ratio of A B in the gas phase (as we distill on, there is slowly more B in the gas phase). That again results in a slowly changing ratio AB in the distillate. If the difference in vapor pressure between the two components A and B is big (in general expressed in the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and, when component A has distilled off, the boiling liquid is enriched in component B. The case of continuous distillation Here we have again a flask charged with the mixture of A and B, which is boiling. On top of the flask is a column, which contains again a mixture of A and B in the gas phase, in a ratio following Raoult's law. The mixture in the boiling liquid is enriched in B, while the mixture in the gas phase is enriched in A. In a continuous distillation, both from the boiling liquid and from the gas phase fractions are withdrawn, at such a speed that the combined ratio of these two fractions is exactly the same as the ratio in the starting mixture. This has as a result, that the ratio of A and B in the system does not change. In this way a stream of enriched component A and a stream of enriched component B is obtained, while the actual distilling mixture does not change ratio, en hence, properties. Moreover, one can supply a stream of crude mixture (which has the same ratio of A and B as the mixture in the still) to the distilling mixture to replenish the liquid, and in that way the system can be run continuously. General improvements Both batch and continuous distillations can be improved by making use of a fractionating column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as long as possible. The column can even exist of small subsystems ('dishes') which all contain an enriched, boiling liquid mixture, all with their own vapour phase. There are differences between laboratory-scale and industrial-scale fractionating columns but the principles are the same. Examples of fractionating columns (in increasing efficacy) include: Laboratory scale distillation Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a still, consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapour is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also ). Simple distillation In simple distillation, all the hot vapors produced are immediately channeled into a condenser which cools and condenses the vapors. Thus, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law. As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C), or to separate liquids from involatile solids. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose. Fractional distillation For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Thus, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component. In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description. More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates. Steam distillation Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive. This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water. Steam distillation of various aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The essential oils are often used in perfumery while the distillates have many applications particularly in skin care. Vacuum distillation Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator. This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure. Air-sensitive vacuum distillation Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used. The Perkin triangle, has means via a series of glass or teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected. Rotary evaporation In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a water aspirator or a membrane pump. Azeotropic distillation Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in a constant-boiling azeotrope which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95% at 78.2 °C. If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically remove or sequester the impurity. For example, to purify ethanol beyond 95%, a drying agent or a desiccant such as potassium carbonate can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well. Short path distillation Short path distillation is a distillation technique that involves the distillate traveling a short distance, often only a few centimeters. A classic example would be a distillation involving the distillate traveling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures. Advantages are that the temperature of the boiling liquid does not have to be much higher than the boiling point of the distilling substance, and the gases only have to travel a short distance while in the gas-phase before they can be cooled again to a lower temperature. Kugelrohr distillation In a kugelrohr distillation a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump. Reactive distillation The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is useful for driving equilibrium reactions such as acid-catalyzed esterification. RCOOH + R'OH ↔ RCOOR' + H2O The reactants, usually carboxylic acids and alcohols, are often high-boiling due to hydrogen bonds; the product, the ester, is usually more volatile. By continually removing the ester product, by le Chatelier's principle, the reaction will proceed to completion. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup. Destructive distillation Destructive distillation involves the strong heating of solids (often organic material) in the absence of oxygen (to prevent combustion) to evaporate various high-boiling liquids, as well as thermolysis products. The gases evolved are cooled and condensed as in normal distillation. The destructive distillation of wood to give methanol is the root of its common name - wood alcohol. Pervaporation Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane. Dry distillation Dry distillation is the heating of solid materials to produce liquid or gaseous products, which may condense into solids. Extractive distillation Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. Flash evaporation Flash (or partial) evaporation is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations. Industrial distillation
Distilled beverages Main article: Distilled beverages Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. As shown in theory, other components than ethanol are collected in the condensate, including water, esters, and other alcohols which account for the flavor of the beverage. For better or worse, distilled beverages such as whiskey and vodka are a part of many people's lives. Gallery | |||||||||
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